Minor fixes for AU280 [work in progress]

terasic_sockit: fix: make video clock also optional as video terminal is optional

antmicro_lpddr4_test_board: fix ethernet rx delay issue

arty: add option to use litespi QSPI controller

Signed-off-by: Karol Gugala <kgugala@antmicro.com>

Signed-off-by: Karol Gugala <kgugala@antmicro.com>

Qmtech Wukong: updates

boards/compat: Skip targets with unsatisfied dependencies.

platforms/ecpix5: Add ULPI signals.

fix ethernet clock (it's a GMII), add FB support over the HDMI connector (hdmi clock set from the resolution)

Fixes #194

Add target for LPDDR4 Test Board

targets/digilent_nexys_video: Add specific Video PLL to give more flexibility on supported Video Timings.

- main_ram mem_map is now directly used by add_sdram when origin is None.
- max_sdram_size/min_l2_data_width are no longer exposed as targets arguments this can
still be used enforced directly in the few cases it is useful.